The present invention relates to a new and improved method for the treatment of boron-containing steel for continuous casting, especially for teeming into small sectional shapes or formats with unregulated nozzles, wherein steel which has been pre-deoxidized with manganese, silicon and, if desired, aluminium, has added thereto elements such as calcium and boron by means of a carrier gas.
It is well known that boron increases the hardenability of steel; it is approximately ten to one hundred times more effective for this purpose than other alloying elements. For many fields of application it is advantageous that boron-containing steels are more easily deformable in the unhardened state than steels where there has been obtained, by means of other alloying elements, the same hardness and strength properties. However, if too much boron is added the steel becomes brittle, and therefore, it is necessary to comply with narrow limits, for instance, boron is usually added in amounts of 0.0008 to 0.0030%. Throughout the disclosure reference to percent shall be understood as meaning percent by weight. However, to obtain an optimum effect within such range, depending upon the composition of the steel particularly upon the carbon content, even more narrow ranges must be positively and predeterminably maintained. In order to also obtain the desired effect it is important that such boron be present as a metal in the steel and not be bound as an oxide or nitride. Therefore, it is necessary that the excess oxygen and nitrogen be stably bound by other elements. The quantities of silicon and manganese which are usually contained in steel are not adequate for this purpose.
During ingot pouring the required deoxidation of the steel is obtained by the addition of large quantities of aluminium, and it must be ensured that also an adequate quantity of aluminium remains in the steel even if the oxygen content was particularly high prior to deoxidation. At the same time by the addition of titanium, zirconium or the like there is stably bound the nitrogen which has dissolved in the steel such that it no longer can react with the added boron. Additionally, there are frequently prescribed for such steels according to prevailing standards or specifications a content of metallic aluminium in a range of 0.020 to 0.040%, whereby for instance there is strived for insensitivity against unintentional overheating during the thermal treatment, especially during hardening.
Since during the continuous casting of steel with an aluminium content exceeding 0.007% there is present the danger that, during the casting operation, the nozzles will clog due to the deposition of aluminium oxide, such steels only can be cast with stopper-regulated, over-dimensioned nozzles. In order to bundle the casting jet, which tends to flutter during the throttling which occurs at the tundish stoppers, it is necessary to use immersible pouring tubes. These immersible pouring tubes likewise must be over-dimensioned, in order to thus compensate the deposition of alumina. Therefore, such steels can only be cast during continuous casting in large sectional shapes or formats, for instance into slabs or blooms.
During the continuous casting of small sectional shapes, such as for instance, billets, there however arise difficulties owing to the sensitivity of the infeed regulation, the relatively high casting speeds which are typical for such sectional shapes and, especially, due to the size of the immersible pouring tubes which must be introduced into the continuous casting mold. These difficulties can cause disturbances in the course of the casting operation. Therefore, small sectional shapes are usually cast with so-called free-flowing, unregulated nozzles, where the throughflow quantity, and thus, the withdrawal speed is determined by the internal diameter of the nozzle. During the casting or teeming operation this should not vary, in particular the nozzle should not clog. With the conventional methods it is therefore necessary to limit the aluminium content of the steel, depending upon composition and temperature, to a maximum of 0.004 to 0.007%. With this low content of aluminium it is not possible to obtain the low content of soluble oxygen which is necessary if the boron is to be added as an alloy in the ladle, and thus, there is to be ensured the requisite narrow range of metallic boron which is dissolved in the steel.
It is known from the continuous casting art, during the continuous casting of boron-containing steels into billets, to infeed the boron in the form of wire into the continuous casting mold, for instance by means of the casting jet of the tundish. However, this is associated with the drawback that the addition is extensively ineffectual and to a degree which cannot be clearly predicted ahead of time, because the boron reacts with the oxygen and nitrogen which has not yet dissolved in the steel, and therefore, becomes ineffectual in terms of improving the hardenability of steel. If, however, the added quantities of boron are increased, in order to compensate for such additional oxidizing loss, then the danger exists that there will be undesirably obtained too high a boron content. With the known addition of boron in wire form there furthermore is present the difficulty that, in order to incorporate a sufficient quantity of boron into the melt, the wire thickness and/or the wire infeed speed must be maintained quite high. However, the need for large wire thickness is associated with the difficulty of handling the wire due to the increased wire stiffness, and the increased infeed speed results in difficult to control inaccuracies in the boron content.
Equally, it is known from practise to deoxidize a steel by blowing-in pulverulent calcium in the form of CaSi or CaC.sub.2 or the like, by means of an inert carrier gas, such as typically nitrogen or argon. More recent investigations have shown that it is possible to cast such treated steel even then with a freeflowing nozzle if it simultaneously also contains increased quantities of metallic aluminium, for instance 0.040%. With this known method which has heretofore preferably been used for ingot casting the largest quantity of the oxygen which has been dissolved in the steel is normally initially bound by the addition of aluminium, and only thereafter the smaller quantity of such oxygen by blowing-in calcium. On the one hand, aluminium is less expensive than calcium, and, on the other hand, it has been recognized that inclusions consisting of calcium aluminate can be more easily removed from the steel than aluminium-free calcium-oxide inclusions.
Furthermore, there is known to the art a method for the continuous casting wherein there are added to a casting ladle and/or a tundish, by means of a transport gas, for instance, oxidation, reduction or neutral gas, different elements for the deoxidation, alloying, cooling and so forth in the form of additives in powder form. In this way there should be possible a uniform distribution of the additives in the melt for the purpose of obtaining a homogeneous cast section. With this prior art method there is not however dealt with the problem of treating boron-containing steels, which particularly should be suitable for casting into small formats or sectional shapes.